Apparatus for writing digital information in a disc-shaped optically readable record carrier

ABSTRACT

An apparatus for recording optically detectable information in a record carrier provided with information areas arranged in accordance with a spiral or concentric track pattern, which areas alternate with synchronization areas each containing the address of the associated information area. Recording is effected with a first modulated laser beam, while a second beam is projected after the first beam for reading the recorded information. The track pattern has previously been provided with a periodic track modulation of a frequency at which the power spectrum of the information to be recorded has a substantially zero value. The apparatus comprises a first and a second filter for filtering a signal corresponding to the track modulation out of the signal derived by detecting the first or second laser beam reflected or transmitted by the record carrier. In series with the second filter there is included a delay network from which a synchronization signal is available. Furthermore, there is provided a phase comparator which, during reading of the synchronization areas, determines the phase difference between the signals obtained from the first and the second filter and which controls the delay network in such a way that the output signal of the delay network is in phase with the output signal of the first filter.

The invention relates to an apparatus for writing information in adisc-shaped optically readable recording medium which comprises asubstrate having a radiation-sensitive information layer withinformation areas arranged thereon in accordance with a spiral orconcentric track pattern. The information areas alternate withsynchronization areas in each of which the address of the associatedinformation area is recorded in an optically detectable manner. Theapparatus comprises a light source, an optical system for directing afirst light beam to the information areas, a write circuit withmodulation means for modulating the light beam, in order to recorddigitally coded information of fixed bit frequency in the informationareas, a first detector for the detection of the radiation of the firstlight beam which is reflected or transmitted by the record carrier, anda read circuit for decoding the radiation detected by the first detectorin order to read the information recorded in the synchronization areas.

Such apparatus is known from Netherlands patent application No. 782859which has been laid open to public inspection and which corresponds tocopending U.S. application Ser. No. 140,409, filed Apr. 14, 1980. Insuch an apparatus the synchronous recording of data is very intricate,because synchronization information is available only when thesynchronization portions are being scanned. The provision of additionalsynchronization tracks or portions is one of the solutions, but thislimits the data storage capacity of the record carrier.

In the U.S. patent application Ser. No. 134,392 filed Mar. 26, 1980, arecord carrier is proposed which does not have this problem but whichcannot always be utilized in an optimum manner for the recording ofdata. It is the object of the invention to provide an apparatus of thetype mentioned in the preamble which enables the record carrier of thelast-mentioned copending application to be utilized in an optimummanner.

To this end the invention is characterized in that the apparatus isadapted to record information in a record carrier whose synchronizationareas and information areas exhibit an optically detectable periodictrack modulation of a frequency for which the power spectrum of theinformation to be recorded substantially exhibits a zero point, forgenerating a clock signal at least during recording, and that theapparatus comprises a second optical system for projecting a secondlight beam onto the track pattern of the record carrier behind the firstlight beam, a second detector for detecting the radiation of a secondlight beam which is reflected or transmitted by the record carrier, afirst and a second band-pass filter tuned to the frequency correspondingto the periodic track modulation, the first and the second band-passfilter respectively being connected in series with the first and thesecond detector, a first adjustable delay network with a control input,an input an an output, of which the input is coupled to an output of thesecond band-pass filter, and on the output of which a clock signal isavailable for synchronization of the write circuit, a phase comparisoncircuit having a first and a second input and an output, of which thefirst input is connected to an output of the first band-pass filter, ofwhich the second input is connected to the output of the delay network,and of which the output is connected to the control input of the delaynetwork via switching means for adjusting the time delay of said delaynetwork in such a way that the signal on the output thereof is in phasewith the signal on the output of the first band-pass filter, and controlmeans for closing the said switching means when the first beam scanssynchronization areas and for opening said switching means when thefirst beam scans information areas.

The apparatus is based on the recognition that the use of the recordcarrier proposed in the said application Ser. No. 134,392 does solve thesynchronization problem, but that the detection of the periodic trackmodulation in the information sectors during recording by means of lightpulses of comparatively high radiation intensities is found to presentpractical problems, especially at higher bit frequencies. In accordancewith the invention use is made of a follower beam--which may also beused for detection and monitoring of the signal being recorded--in orderto read the clock signal during recording. When the synchronizationportions are read with the first laser beam, the delay network is eachtime adjusted so that the signal detected by the follower beam is insynchronism with the signal detected by the first beam. The delayedsignal is then used as clock signal during recording. The invention isdescribed in more detail with reference to the drawings, in which.

FIG. 1 shows one embodiment of a record carrier to which the inventiveprinciple may be applied, FIG. 1a being a plan view of the recordcarrier, FIG. 1b showing a part of a track 4 of the record carrier on anenlarged scale, and FIG. 1c showing a synchronization area of said parton an enlarged scale.

FIG. 2 shows a small part of the cross-section taken on the line II--II'in FIG. 1a.

FIG. 3, in FIGS. 3a through 3d, schematically shows a cross-section inthe longitudinal direction through a part of the track 4, FIG. 3ashowing such a cross-section for a blank prepared disc in accordancewith a known technology, FIG. 3b showing said cross-section of FIG. 3aafter information has been recorded in the information area 9, FIG. 3cshowing such a cross-section of a blank prepared disc in accordance withthe invention, FIG. 3d showing the cross-section of FIG. 3c afterdigital information has been recorded, FIG. 3e schematicallyrepresenting the signal obtained when reading the part of the track 4shown in cross-section in FIG. 3d, and FIG. 3f schematicallyrepresenting a plan view of a part of the track 4 after digitalinformation has been recorded in a manner other than in FIGS. 3b and 3d,

FIG. 4 shows the random power spectra of three digital informationsignal modulations,

FIG. 5 is a diagrammatic representation of said modulations,

FIG. 6 in FIG. 6a schematically represents an apparatus formanufacturing a record carrier in accordance with FIG. 3c, FIG. 6bschematically represents an apparatus for recording information in therecord carrier of FIG. 3c, and FIG. 6c represents an apparatus forreading an inscribed record carrier,

FIG. 7 shows a number of examples of a periodic track modulation inaccordance with the invention.

FIG. 8a illustrates the principle of a read section of an apparatus forreading and/or recording a digital signal on a record carrier inaccordance with the invention, FIG. 8b representing the frequencyspectrum of the signal detected by the detector 27,

FIG. 9a shows an apparatus in accordance with FIG. 8a, which is alsosuitable for generating a radial tracking signal, FIG. 9b representingthe frequency spectrum of the signal detected by the detector 27.

FIG. 10 shows a variant of the apparatus of FIG. 9a,

FIG. 11a shows an apparatus in accordance with FIG. 9a adapted to arecord carrier with a radial track modulation of substantially the sameperiod as the periodic track modulation, FIG. 11b representing thefrequency spectrum of the signal detected by the detector 27,

FIG. 12 shows an apparatus adapted to a record carrier with a radialtrack modulation of the same period as the periodic track modulation,and

FIG. 13 shows a part of an apparatus for recording an information signalon record carrier in accordance with the invention for generating aclock signal during recording, use being made of an auxiliary laserbeam.

FIG. 1 shows one embodiment of a record carrier to which the inventiveprinciple may be applied, FIG. 1a showing a plan view of this recordcarrier, FIG. 1b showing a part of a track 4 of said record carrier onan enlarged scale, and FIG. 1c showing a synchronization area of saidpart on en enlarged scale. The record carrier body 1 is provided with aspiral track 4. Track 4 is divided into a multitude of sectors 7, forexample 128 per revolution. Each sector 7 comprises an information area9, intended for recording digitally coded information, and asynchronization area 8.

In order to ensure that the digital information is recorded in anaccurately defined path, the track 4 is employed as a servo track. Forthis purpose the information area 9 of the sectors 7 have an amplitudestructure shown in FIG. 2. This FIG. 2 shows a small part of thecross-section taken on the line II--II' in FIG. 1a and thus show anumber of adjacent track portions, specifically information areas, ofthe servo track 4. The direction of the servo tracks 4 is thusperpendicular to the plane of drawing. The servo tracks 4, in particularthe information areas 9, are thus formed by grooves in the substrate 5.In this way it is possible to make a radiation beam, which is directedat the record carrier in order to record digital information, accuratelycoincide with said servo track 4, in other words to control the positionof the radiation beam in a radial direction via a servo system whichemploys light reflected by the record carrier. The measurement of theradial position of the radiation spot on the record carrier may be inaccordance with systems similar to those employed in optical recordcarriers provided with a video signal and as inter alia described in"I.E.E.E. Transactions on Consumer Electronics, Nov. 1976, page 307.

For recording of digital information the record carrier body is providedwith a layer of a material 6 which, if exposed to suitable radiation, issubject to an optically detectable change. In principle only theinformation portions 9 of the sectors need be provided with such alayer. However, for reasons of manufacturing technology it is simpler toprovide the entire record carrier surface with such a layer. This layer6 may for example comprise a thin layer of a metal such as tellurium.This metal layer can be melted locally by laser radiation ofsufficiently high intensity, so that locally this information layer 6 isgiven a different reflection coefficient, as a result of which thereflected beam is amplitude-modulated in accordance with the recordedinformation when an information track thus inscribed is scanned by aread beam.

Alternatively, the layer 6 may be a double layer, for example ofaluminium on iron, which react chemically to incident radiation. At thelocation where a high-power radiation beam is incident on the disc FeAl₆is formed, which is a poor reflector. A similar effect is obtained inthe case of a double layer of bismuth on tellurium, Bi₂ Te₃ beingformed. It is also possible to employ a single layer of tellurium.

As with the aid of the servo track in the form of a groove in thesubstrate 5 the write radiation spot is made to coincide accurately withsaid servo track, in particular when an information area is beingscanned, the digital information modulating the write beam is recordedexactly in the information area coinciding with said servo track.

As is apparent from the foregoing the record carriers intended for theuser, in which the information areas do not yet contain information,have a groove structure in said information areas within the sectors.Moreover, within each sector such a record carrier has a synchronizationarea 8 in the form of an optically detectable relief structure. FIG. 1bon an enlarged scale shows a part of a track 4 with a sequence of anumber of information areas 9 and synchronization areas 8. In this casethe synchronization areas 8 comprise a relief structure constituted by asequence of recesses alternating with intermediate areas.

The depth of the recesses in this structure of the synchronization areais greater than the depth of the servo track in the information area 9.This depth of the recesses is selected in accordance with generaloptical rules and depending on the shape of said recesses in theselected read system in way such that an optimum read-out of theinformation represented by the structure is obtained. In the case of aread system in which the radiation beam reflected by the record carrieris detected by a single photo-detector, 1/4λ may be selected as depthfor the recesses, λ being the wavelength of the radiation used. If forthe depth of the servo track in the information area 9 the value 1/8λ orless is selected, this servo track will hardly affect the amount oflight detected by the detector.

In order to further illustrate the structure of the synchronizationarea, FIG. 1c again shows such a synchronization area on an enlargedscale, the information layer 6 being omitted for the sake of simplicity.Such a synchronization area 8 has two portions, namely an indicationportion 10 and an address portion 11. The address portion 11 containsall the information required for controlling the recording process. Whenrecording digital information, this information is converted into aso-called word-organized bit series. The address portion containsinformation about the word organization, so that during recording thelocation of the bit words is defined and during reading the bit wordsare suitably decoded. Furthermore, address portion 11 containsinformation about the relevant track number. This information is in theform of a relief structure in accordance with a digital modulationtechnique suitable for the recording medium. Since in addition to theservo track in the form of a groove in the information portions 9, therecord carrier already contains, in the synchronization area, all theinformation required for positioning information in the form of abit-word-organized bit series in said information areas, therequirements imposed on the write and read apparatus employed by theuser may be less stringent. Furthermore, since this fully prerecordedinformation is in the form of a relief structure, the record carrier isparticularly suitable for mass-production, enabling the costomarypressing techniques to be used.

FIGS. 3a through 3d, schematically represents a part of such a servotrack 4 in a cross-section in the longitudinal direction of the servotracks 4 with a part of the synchronization area 8 and a part of theinformation area 9 with:, FIG. 3a showing such a cross-section of ablank prepared disc using a known technique, FIG. 3b showing such across-section after digital information 14 has been recorded in theinformation area 9, FIG. 3c showing such a cross-section of a blankprepared disc provided with clock information in accordance with theinvention, and FIG. 3d representing the cross-section of FIG. 3c afterinformation 14 has been recorded in the information area 9. FIG. 3eschematically represents the signal obtained when reading the part ofthe track 4 shown in cross-section in FIG. 3d and FIG. 3f schematicallyshows a plan view of a part of the track 4 after information has beenrecorded in a manner other than represented in FIGS. 3b and 3d.

The prepared disc is provided with a servo track 4, formed in asubstrate 5, for example by means of a laser beam. By modulating theintensity of the laser beam it is then possible to form a reliefstructure of "pits" 13 containing information in the synchronizationarea 8. Subsequently, the entire disc, including for the sake ofsimplicity, the portion of the record carrier 1 outside the grooves 4,may then be coated with the reflecting information layer 6. In therecord carrier thus prepared information can be recorded in theinformation area 9 by forming holes 14 in the reflecting informationlayer 6, for example by means of a laser beam. Such an inscribed recordcarrier is shown in FIG. 3b. When information is written, i.e. the holes14 are formed, and when the information is read, for example by means ofa laser beam, it is of importance that this information writing orreading process is synchronized with the aid of a clock signal, aboutwhich the synchronization areas 8 may contain information, In order toensure that during writing and reading a suitable synchronous clocksignal is continuously available, i.e. also during writing or reading inthe information areas 9, the servo groove 4 is in accordance with theinvention, provided with a structure which produces a modulation of thelight reflected by the information carrier when the servo track 4 isfollowed during reading or writing.

However, this structure should be such that it does not disturb theread-out of information. This is accomplished in the manner explainedwith reference to FIGS. 4 and 5, FIG. 4 representing the random powerspectra of three possible binary information-signal modulations and FIG.5 being a diagrammatic representation of said modulations.

The reference a in FIG. 5 designates a modulation known by the name of"biphase" modulation. In this technique applied digital signal isconverted into a binary signal in which a logic "one" of the applieddigital signal is positive during the time interval T/2 and negativeduring the next time interval T/2, T being the bit length of the applieddigital signal. A logic "zero" yields exactly the opposite binarysignal, i.e. negative for the time interval T/2 and positive for thenext time interval T/2. This modulation technique yields a binary signalwhich has a power spectrum represented by a in FIG. 4. The frequency fothen corresponds to 1/T.

The reference b in FIG. 5 represents the modulation known by the name of"Miller" modulation. The binary signal generated by means of thismodulation has a transition halfway a logic "one" of the applied digitalsignal and at the transition of two consecutive logic "zeros". Thefrequency spectrum of the binary signal obtained by means of thismodulation technique has the designation b in FIG. 4.

Finally, the reference c in FIG. 5 represents a modulation known by thename of "quadphase" modulation, the applied bit series of the digitalsignal first of all being divided into consecutive groups of two bits.From each two-bit group having a duration of 2T a binary signal isderived which in a first time interval T has the same variation as theoriginal two bits and in the next time interval T an inverse variation.The bit combinations 11, 00, 01 and 10 which are possible are thusconverted into the bit combinations 1100, 0011, 0110 and 1001respectively. The binary signal obtained by means of this modulationtechnique has a frequency spectrum represented by c in FIG. 4.

It is evident from FIG. 4 that these modulation techniques have thecommon property that the resulting binary signal exhibits no strongfrequency components at comparatively low frequencies, for examplefrequencies below 0.2 fo. This is very useful when an optical recordcarrier is used with the associated write and read systems. As statedpreviously, such systems employ both a servo control in order to keepthe scanning spot accurately focussed on the record carrier and a servocontrol which controls the radial position of the scanning spot so as toensure that the scanning spot accurately coincides with the informationtrack. Since the control signals required for these servo controls arederived from the radiation beam which is reflected by the record carrierand which is also modulated by the relief structure of thesynchronization area, it is essential that the frequency spectrum of thebinary signal stored in the address portion does not contain any strongfrequency components within the frequency band intended for the controlsignals. FIG. 4 thus shows that the frequency band below approximately0.2 fo is suitable for such control signals. The control signals for theservo systems may for example extend to a maximum frequency of 15 kHz.If for the frequency fo=1/T, for example, the value 500 kHz is selected,it will be evident from FIG. 5 that the binary signals a, b or c onlyexhibit very weak frequency components at frequencies of 15 kHz andlower. Furthermore it is apparent from FIG. 4 that at the frequency 2 foand in the case of modulation method c the spectrum also has zero pointsat the frequency fo, Thus, it is possible to provide the record carrierwith a clock structure of the frequency 2 fo without interference withthe information signal. Zero points at the frequency 2 fo also occur inthe case of other modulation methods.

When quadphase modulation (modulation c) is used and also in the case ofsome other modulation methods, the frequency fo is highly suitable forthis purpose, said frequency corresponding to the bit frequency 1/T, sothat this quadphase modulation becomes very attractive. Also in the caseof modulation method b a structure with the frequency fo may be used insome cases because the components of the spectrum of modulation b arecomparatively small at that frequency. Furthermore, it is theoreticallypossible to give the structure a modulation corresponding to a frequencyhigher than 2 fo, but in practice this is generally not feasible.Indeed, in view of a maximum information density, the dimensions of thepits 13 and 14, which at a specific speed of rotation of the disc 1 atleast correspond to a bit length of 1/2T, are selected nearest theresolution of the write/read system used, so that the surface structurecorresponding to frequencies higher than 2 fo will hardly be detectable.By means of special modulation techniques it is also possible to obtainzero points in the power spectra at frequencies other than fo or 2 fo,for example at 1/2 fo.

FIG. 3c shows a cross-section of a record carrier in accordance with theinvention corresponding to the cross-section of FIG. 3a, whose surfaceat least at the location of the track 4 has been provided with a reliefstructure having a height d. One way of realizing this structure is tomodulate the laser by means of which the synchronization area 8 and thegroove 4 of the information area 9 is formed. In the present examplethis has only be done in the synchronizaton area 8 between the pits 13by limiting the intensity of the laser beam. However, in principle it isalso possible to provide the bottoms of the pits with a reliefstructure. As shown in FIG. 3d the disc in accordance with the inventioncan also be provided with information by forming holes 14 in thereflecting layer 6 covering the relief structure. FIG. 3c shows anexample of a signal obtained when reading a relief structure inaccordance with FIG. 3d. This signal exhibits minima at the location ofthe pits 13 or the holes 14 and an amplitude modulation (d in FIG. 3c)corresponding to the modulation structure with the frequency fo at themaxima. The modulation structure of the bottoms of the holes 14 hardlycontributes to the signal, because it hardly reflects any light owing tothe removal of the reflecting layer 6. In this respect it is to be notedthat it is for example also possible to provide a non-reflecting layer 6on a reflecting substrate 5, which layer is locally removed. As a resultof this the modulation of the frequency fo will be read satisfactorilyat the very locations 14 where the non-reflecting layer has beenremoved.

In FIGS. 3a-3d the pits 13 or the holes 14 are shown as continuous holesor pits, i.e. in the case of more than one bit as an elongate slothaving a length corresponding to the number of consecutive bits.However, it is alternatively possible to provide a separate pit or holefor each bit. FIG. 3f illustrates this and shows a track 4 in which theclock modulation structure is represented by different types ofhatching. In the synchronization area 8 the pits 13 may for example beformed in the centre of the maxima or minima of the structure and arealso coated with a reflecting layer 6, which is symbolized by thelatching through said pits 13. In the information portion 8 theinformation holes 14 may be formed in the reflecting layer 6 at themaxima and minima of the clock information structure. Alternatively--asis represented by the information area 9 in FIG. 3f--holes 14' may beformed at the zero points of the information structure. In this respectthe location of the pits 13 or holes 14 is not essential, provided thatthe phase relationship with the clock information structure is fixed andknown. Neither is the shape of the information structure of greatsignificance. Instead of the rectangular shape shown in FIG. 3 it maywell have a sinusoidal shape, which is readily possible in the case ofmanufacture by means of a modulated laser beam. It is of importance onlythat said clock synchronization structure exhibits a frequency componentwhich can readily be detected at the frequency fo or 2 fo and whichexhibits no strong components within the spectrum of the synchronizationor digital-information signal recorded or to be recorded. This isgenerally the case when the clock information structure d has afundamental frequency fo or 2 fo with higher order harmonics only; thenext harmonic is then 2 fo or 4 fo, which, as shown in FIG. 4, fallsbeyond the part of the information spectrum that is of interest.

In order to illustrate how structures in accordance with FIG. 3 can berealized FIG. 6a schematically shows an apparatus for manufacturing arecord carrier of FIG. 3c, FIG. 6b an apparatus for recordinginformation in the record carrier of FIG. 3c, and FIG. 6c an apparatusfor reading such an inscribed record carrier.

In the apparatus of FIG. 6a the beam 16 from a laser 15 is projected ata rotating disc 1 via for example an intensity modulator 57, a mirror 17and a focusing optic 18, in order to form the spiral groove 4 (FIG. 1).The laser 15 is controlled by a circuit 20 for pulsating the laser 15 soas to form the pits 13 (FIG. 3) in the synchronization area 8. Themodulator 57 is controlled by a source 19 having a frequency fo (or 2fo) in order to realize a clock modulation structure in the groove 4.Alternatively, it is possible to modulate the laser 15 itself. The disc1 is driven by a motor 21 whose speed is controlled by a servo control,which may for example comprise a tachogenerator 22, a speed-referencesource 24, and a servoamplifier 23. In order to ensure that therecording areas 8 are situated at the correct location on the disc inthe track 4 and, as the case may be, to obtain a correct tangentialdistribution of the modulation fo on the disc, the circuit 20 and, asthe case may be, the source 19 of the frequency fo may be locked to theservo control. Furthermore the circuit 20 is controlled by the source 19in order to guarantee a correct phase relationship of thesynchronization pits 30 with the clock modulation structure. After thisprocess the disc 1 may be provided with the layer 6.

FIG. 6b schematically represents an apparatus for providing the prepareddisc 6 with information and simultaneously reading the clock modulationstructure. This apparatus comprises the rotating disc 1, and a laser 15whose beam 16 is projected, via a semitransparent mirror 17 and afocusing optic 18, onto the disc 1. A reflected beam 30 is detected bymeans of a cell 27, for example a photodiode, and converted into anelectric signal from which the component of the frequency fo (or 2 fo)is extracted by means of the band-pass filter 28, That component isproduced mainly by the clock modulation structure formed in the track 4.As the case may be, this signal may also be applied to a phase-lockedloop 29, which improves the filtration, increases the constancy of theclock signal and, as the case may be, compensates for brief signaldropouts. The clock signal is then available on output 31. Data can berecorded by pulse modulation of the laser beam 16, directly by includinga modulator in the beam or, as is shown in FIG. 6b, by modulating thelaser 15 itself with a write modulator circuit 25, to which theinformation is applied via an input 26 and which is synchronized withthe clock signal on output 31.

The light-sensitive element 27 and a read circuit 30 are used to recoverthe information contained in the synchronization portions from thereflected beam 60, which information appears on an output 32. The readcircuit 30 may also be synchronized with the clock signal on output 31.The recovered information may be used to synchronize the circuit 25 andto locate the correct position on the disc. This information is alsoused in a servo control, not shown in FIG. 6b, for radially positioningthe optic 18 and the mirror 17, for inscribing the desired portion ofthe track 4 and for controlling the drive of the disc 1, which issymbolically represented by the dashed line 62 in FIG. 6b.

Furthermore, the apparatus may be provided with a tracking circuit 33which derives a tracking signal from the signal supplied by the detector27 in order to keep the beam 16 on the track 4 by controlling the anglerelative to the beam 16 of the mirror 17, which is symbolized by thedashed line 61 in FIG. 6b.

FIG. 6c shows an apparatus for reading an inscribed disc 1, whichapparatus is generally combined with that of FIG. 6b. The apparatusagain comprises a laser 15, whose beam 16 is projected onto the disc 1via a mirror 17 and the optic 18. The reflected beam 60 is detected witha photodiode 27 and the resulting electric signal is passed through aband-pass filter 28 having a pass frequency fo and a phase-locked loop29 tuned to the frequency fo, so that the clock signal of the frequencyfo (or 2 fo) is available on output 31. The information recorded on thedisc is decoded from the electric signal supplied by the photodiode 27by means of the read circuit 30, so that on an output 32 thereof thedigital information and the information contained in the synchronizatonareas 8 is available. This read circuit is synchronized by means of theclock signal on output 31. In addition, a tracking signal may be derivedfrom the beam detected by photodiode 27 by means of a tracking circuit33, in order to control the mirror 17 in such a way that the beam 16exactly follows the track 4. The disc drive motor 21 may be included ina servo control, for example comprising a tachogenerator 22, a referencesource 24, and a servo amplifier 23, in order to control the speed,which control may be locked to the read circuit 30. Furthermore, theapparatus also comprises a control mechanism 35 for moving the optic 18together with the mirror 17 and the detector 27--the complete mechanismbeing designated 36 in FIG. 6c--in a radial direction, so that adesired, a specific part of the disc can be read, controlled byinformation applied to an input 37 of the control mechanism 35 and bythe information produced by the synchronizaton areas and available onoutput 32 of the read circuit 30.

The clock information structure which is or has been recorded in track 4may take various forms. FIG. 7 shows a number of examples thereof. FIG.7a schematically represents a track 4 in which the clock information isformed by a height variation--symbolically represented by theinterrupted hatching--for example by modulating the intensity of thelaser beam that writes the track 4, FIG. 7b shows the track 4 in whichthe clock information is formed as a width variation of the track 4, forexample by modulation of the laser-beam focusing, for which for examplethe objective 18 (FIG. 16a) may be controlled by means of the device 59(FIG. 6a)--whilst a combination of width and depth variations is alsopossible, which in practice will frequently be the case when theintensity or focusing of the laser beam is modulated--and FIG. 7c showsthe track 4 in which the clock information takes the form of a radialvariation of the position of the track 4, for which purpose for examplethe angle of the mirror 17 (FIG. 6c) relative to the beam 16 can bemodulated by means of the device 58. All the variations shown then havea period length Lo which is equal to Lo=V/f, where V is the tangentialspeed of the disc 1 at said location and f the frequency of the desiredclock signal, which frequency f corresponds to a zero point in therandom frequency spectrum of the data to be recorded, for example thefrequency fo (FIGS. 4c and 5c ) in the case of "quadphase" modulation.

One of the possibilities of obtaining a tracking signal is by providinga radial "wobble" in the groove-shaped track, for example by controllingthe mirror 17 (FIG. 6a), i.e. a for example sinusoidally varying radialexcursion with a wavelength on the disc which during playback at thenormal speed produces a light intensity variation detected by thedetector 27 (FIG. 6), whose frequency is situated outside the spectrumof the data, i.e. for example below the frequency 0.2 fo (FIG. 4). Forexample by synchronous detection, a measure of the deviation of thecentre of the detector relative to the centre of the track 4 may bederived from said signal component. Such a radial wobble may be combinedwith a clock modulation structure, for example the clock modulationstructure shown in FIG. 7a, which combination is shown in FIG. 7d. Aspecial combination is obtained when the wobble on the disc has awavelength equal to that of the clock modulation structure with a fixedphase relationship, which makes synchronous detection superfluous.

FIG. 7e shows such a structure, a depth modulation structure(represented by alternately hatched and non-hatched areas) in track 4being combined with a radial positional variation which is 90° shiftedrelative thereto (i.e. a quarter of the period of said structure), whichstructure can be produced with the apparatus of FIG. 6a by modulatingthe angle of the mirror 17 relative to the beam 16 with the aid of thedevice 58. If the depth modulation structure is then selected so thatthe "shallow" parts of these modulations coincide with the surface ofthe disc-shaped record carrier 1, the servo track 4 will take the formof a sequence of radially asymmetrical pits which are tangentiallyspaced from each other by distances equal to the said distance Lo. FIG.7f shows an example of such a track 4.

FIG. 8a illustrates the principle of the read section of an apparatusfor writing data in or reading data from a record carrier in accordancewith the invention, the frequency spectrum of the signal I detected bythe detector 27 being shown in FIG. 8b. The apparatus comprises aphotodetector 27, along which the track 4 is passed. The signal which issupplied by the detector 27 has a spectrum as shown in FIG. 8b, in thepresent example with the spectrum of a quadphase modulated signal Sd anda clock signal Sc. The clock signal Sc is extracted with the aid of aband-pass filter 28, preferably followed by a phase-locked loop 29. Theclock signal Sc is available on output 31. The digital signal Sd, i.e.the signal recorded in the synchronization area 8 and during reading thesignal recorded in the synchronization area 8 and in the informationarea 9, is detected with a read circuit 30 which is synchronized withthe clock signal Sc. The data signal read is available on output 32.Furthermore, a radial tracking signal can be derived from the signalfrom the detector 27. When information is recorded in information areas9 the circuit 30 only detects the information contained in thesynchronization areas 8, which together with the clock signal Sc is thenapplied to the write circuit 25 in order to modulate the beam of a writelaser 15.

When a low-frequency radial wobble is used in order to obtain a radialtracking signal, the apparatus of FIG. 9a may be used, FIG. 9b showingthe frequency spectrum of the signal detected by the detector 27. When atrack 4 with a radial wobble is read it is advantageous to employ aphotodetector 27 which is divided in two sections a and b along an axialline. A differential amplifier 40 or equivalent means forms thedifference of the signals detected by sections a and b a summingamplifier 41 or equivalent means provides the sum of said signals. Thefrequency spectrum (FIG. 9b) again contains the spectrum of thequadphase modulated signal Sd, the clock signal Sc and the low-frequencysignal Sw produced by the wobble. In the sum signal the wobble manifestsitself as an amplitude modulation with the clock signal Sc as carrierwave, which in FIG. 9b is represented as side bands Sc-w and Sc+w, whichside bands have an amplitude equal to zero when the detector 27 exactlyfollows the centre 45 of the track 4. Filtering the sum signal with theband-pass filter 28 yields the clock signal Sc and, if the filter is nottoo narrow-banded, also said side bands. The output signal of saidband-pass filter 28 is applied to the phase-locked loop 29 and on anoutput 31 thereof the clock signal Sc is available. The output signal ofthis band-pass filter 28 is also applied to a synchronous demodulator 42together with the clock signal Sc. This demodulator then produces themodulation Sw.

The frequency of the radial wobble is recovered from the differencesignal from amplifier 40 with the aid of band-pass filter 38 andphase-locked loop 39, which frequency together with the output signal ofthe synchronous detector 42 is applied to a synchronous detector 43. Onthe output 44 thereof the modulation of the wobble signal Sw is thenavailable, which may be used as radial tracking signal and isrepresentative of the deviation of the detector 4 relative to the centreof the track 4, which in FIG. 9a is represented by the dashed line 45.Said radial tracking signal can then control the mirror 17 as issymbolically represented in FIGS. 6b and 6c.

The data contained in the track 4 is then recovered from the sum signalon the output of amplifier 41 in a similar way as in the apparatus ofFIG. 8a. In respect of information recording similar steps may beapplied as in the apparatus of FIG. 8a, which also valid for theapparatus of FIG. 10, FIG. 11a and FIG. 12.

FIG. 10 shows a variant of the apparatus in accordance with FIG. 9,which yields a better signal separation. The detector 27 has also beendivided in accordance with a tangential line, so that four quadrants a,b, c and d are obtained, the sections a, b and c, d respectively beingsituated on either side of the tangential line, and the sections a, cand b, d respectively being situated on either side of the radial line.An amplifier 41 or equivalent means determines the sum of the signalsgenerated by the sections a, b, c and d, so that this amplifier isspecifically sensitive to intensity variations of the beam reflected bythe track 4, i.e. to the data signal Sd, an amplifier 421 determines thedifference between the sections a+b and c+d situated on either side ofthe tangential line, so that said amplifier 421 is particularlysensitive to variations of the track 4 in a tengential direction, i.e.to the clock signal Sc, and an amplifier 46 determines the differencebetween the sections a+c and b+d situated on either side of the radialline, so that this amplifier is particularly sensitive to variations ofthe track 4 in a radial direction, i.e. to the signal Sw correspondingto the wobble.

In a similar way as in the apparatus of FIG. 9a the clock signal Sc isrecovered from the output signal of amplifier 46 by means of band-passfilter 28 and phase-locked loop 29 and the frequency of the wobblesignal Sw by means of band-pass filter 38 and phase-locked loop 39. Theoutput signal of the band-pass filter 28, which contains the wobblesignal Sw as an amplitude modulation of the clock signal Sc, is detectedsynchronous with the clock signal by means of synchronous detector 42and yields the wobble signal Sw whose amplitude variation represents thedeviation of the detector 27 relative to the centre 45 of track 4. Saidsignal Sw is detected synchronously with the output signal ofphase-locked loop 39, i.e. the wobble frequencies, by means ofsynchronous detector 43, so that the radial tracking signal appears onoutput 44. The data signal is recovered from the output signal ofamplifier 41, synchronised by the clock signal Sc, by means of the readcircuit 30.

Mathematically, the operation of the apparatus of FIGS. 9a and 10 inrespect of the recovery of the radial tracking signal may be explainedas follows. The signal I detected by the detector 27 is a product of theclock modulation, the wobble modulation and the radial tracking error,which (when ignoring the data signal) may be expressed as:

    I=Ar sin (w.sub.w T) sin (w.sub.c t)

where Ar is a function of the tracking error, w_(w) the angularfrequency of the wobble signal Sw, w_(c) is the angular frequency of thepilot signal Sc, and t the time. Synchronous detection with the pilotsignal Sc yields the term Ar sin (w_(w) t) and subsequent synchronousdetection with the wobble frequency w_(w) yields the signal Ar.

FIG. 11a shows a read section of an apparatus for reading data from atrack 4 with a clock modulation structure and a wobble for deriving aradial tracking signal, the frequency of the wobble signal Sw beingsubstantially equal to the frequency of the clock signal Sc, and FIG.11b shows the frequency spectrum in which Sd represents the data signaland Sc-w the term having a frequency equal to the difference between thefrequencies of the clock signal Sc and the wobble signal Sw, whichdifference is for example 30 kHz, said term being obtained in that thephotodiode 27 receives the product of the wobble modulation and theclock modulation. As a result of this, said term is situated in thelow-frequency part of the spectrum and is hardly disturbed by thedigital information. The amplitude of this term constitutes the radialtracking signal. The amplitude is zero if the centre line 45 of thetrack is followed exactly. The wobble then yields a term of double thedifference frequencies, which term is not used, and a term with thewobble frequency itself.

The apparatus, in a similar way as the apparatus of FIG. 10, comprisesan amplifier 41 for supplying the sum of the signals supplied bysections a, b, c and d of photodiode 27, from which sum the term of saiddifference frequency is extracted by means of the band-pass filter 48.With the aid of a synchronous detector 43, to which said differencefrequency is applied, this term is demodulated and, as the case may bevia a low-pass filter 49, the radial tracking signal appears on output44.

The clock signal Sc is obtained in a similar way as in the apparatus ofFIG. 10 by determining the difference between the signals supplied bythe two radial halves a+c and b+d of photodiode 27 with amplifier 46 andapplying said difference to a phase-locked loop 29 after filtration withband-pass filter 28. In a similar way as in the apparatus of FIG. 10 thewobble signal Sw is derived by determining the difference between thesignals supplied by the two axial halves a+b and c+d of photodiode 27with amplifier 421 and applying this to a phase-locked loop 39 via aband-pass filter 38. The difference frequency applied to the readcircuit detector 43 is obtained by applying the clock signal S_(c) thusobtained and the wobble signal Sw to a synchronous detector 42, afterwhich the resulting signal of said difference frequency is applied tosynchronous detector 43 via band-pass filter 47.

With the read circuit 30, synchronized with the clock signal Sc, thedata signal can be recovered from the output signal of amplifier 41.

If the frequency of the wobble signal Sw is selected to equal thefrequency of the clock signal, it will be evident from FIG. 11b that theterm with the difference frequency directly constitutes the DC trackingsignal. This tracking signal can then be obtained without synchronousdetection.

The phase difference between the two track modulations should be unequalto zero, because only one modulation can be distinguished when the twomodulations are in phase. It is found that 90° is an optimum phasedifference.

FIGS. 7e and 7d show such a structure, which can be read with the simpleread circuit of FIG. 12.

In the apparatus of FIG. 12 the photodiode 27 is divided into two radialhalves a and b for an optimum detection of the clock signal Sc, which isobtained on output 31 by determining the difference between the signalssupplied by the two halves a and b with amplifier 46, by filtering saidsignal with band-pass filter 28 and applying it to the phase-locked loop29. By filtering the output signal of amplifier 46 with a low-passfilter 49 the radial tracking signal is directly available on output 44.The digital signal is recovered from the difference signal with readcircuit 30, which is synchronized with the clock signal Sc.Alternatively, it is possible to recover the data signal and thelow-frequency tracking signal from the sum of the two halves.

In respect of the tracking during the recording of data signals theapparatus in accordance with FIGS. 8a through 12 may be extended with adevice modulating a laser beam 16, which device is synchronized with theclock signal Sc and the signal read from the synchronization areas, ashas been explained with reference to FIG. 6b.

In the foregoing it has been assumed that one detector 27 is used fordetecting the reflected beam 16 (FIG. 6). Especially at high bitfrequencies it may be problematic, when recording data in the informatonareas 9 with a laser beam which is comparatively powerfull relative tothat used for reading, to recover the clock information from the beamwhich is reflected between every two write pulses. For that reason afollower laser-beam may be employed to detect the recorded data and inthat case the apparatus of FIG. 13 may be used for writing information.In such an arrangement the track 4, which travels in the direction ofthe arrow 63 relative to the detector 27, is scanned by aninformation-writing beam 16a and a follower beam 16b. The two beams can,for example, be obtained by means of a beam splitter 68, mirrors 70a and70b and optical systems 18a and 18b. In order to modulate the beam 16a,a modulator may be arranged in the beam 16a. The apparatus comprises aphoto-diode 27 and with respect to read out of data signals and trackingsignals fully corresponds to the apparatus shown in any of the FIGS. 8a,9a, 10, 11a or 12a. Furthermore, the apparatus comprises a photodiode 50for detecting the reflection of the follower beam 16b which is projectedonto the track at some distance behind the beam 16a. During the readprocess, and also when the synchronization areas 8 are being read, theclock signal Sc is obtained by applying the signal detected byphotodiode 27 to the phase-locked loop 29 via an amplifier which forsimplicity is not shown in this Figure (for example 46 in FIG. 11a) anda band-pass filter. In addition, in particular during the writingprocess, the clock signal is also recovered in a similar way from thesignal detected by photodiode 50, as the case may be via a band-passfilter 500 and via a phase-locked loop 501, but this signal is delayedrelative to the clock signal obtained via photodiode 27. Via a delaydevice 51, the output signal is applied to output 31. The phase of thedelayed clock signal is then compared with the phase of the clock signalobtained by means of the photo diode 27 in phase comparator 52 and viaswitch 53 the delay device 51 is adjusted so that the clock signal fromphotodiode 50, which has been delayed via delay device 51, is in phasewith the signal obtained via photodiode 27. During the read-out of thesynchronizaton areas 8 switch 53 is closed and the delay device 51 isadjusted so that the clock signal from photodiode 50, which has beendelayed by said delay device 51, is in phase with the clock signalobtained via photodiode 27. During the recording of data in theinformation areas 9 switch 53 is open and the clock signal is recoveredfrom the reflected auxiliary beam 16b via photodiode 50 and is delayedwith the delay device 51 by the time adjusted during the read-out of thesynchronization areas 8. The switch 53 is operated on command of thesynchronization signals read from the synchronization areas by the readcircuit 30.

In this respect it is to be noted that writing information with unitpits, i.e. the information is recorded with separately detectablechanges in the surface structure of the record carrier, as is shown inFIG. 3f, yields a frequency component at the frequency 2 fo in thespectrum (FIG. 4) of the signal being read. This need not be a problemfor the use of a clock modulation structure, because this clockmodulation, if it has a frequency equal to 2 fo, may be used whenrecording information, and if during recording a correct phaserelationship with the clock signal is maintained during read-out it willcoincide, with the component 2 fo as a result of the use of unit pits.When quadphase modulation is used (FIGS. 4c and 5c) the clock signalwill have a frequency equal to fo and in that case said component of thefrequency 2 fo is not disturbing.

The invention is not limited to the embodiments shown, which relate to adata storage medium with a subdivision into sectors. The invention mayalso be used in prepared record carriers for the storage of digitallycoded, audio, video and other information in more or less continuousinformation areas.

Furthermore the invention is not limited to record carriers in which therecorded information is detected via reflection of the laser beam, butmay also be employed in record carriers where the recorded informationis detected by detecting the radiation transmitted by the recordcarrier.

Although the description with reference to the Figures is based on theuse of laser beams, it is alternatively possible, in particular duringreading, to employ focused non-coherent light beams.

What is claimed is:
 1. An apparatus for writing information in aoptically detectable form on a recording medium having a substrateprovided with a radiation-sensitive information layer and withinformation areas arranged in a spiral or circular track pattern andalternating with synchronization areas, said synchronization areas eachcontaining an optically detectable address of an associated informationarea, said synchronization and information areas having an opticallydetectable periodic track modulation which is representative of a clocksignal and which is of a frequency at which the power spectrum of theinformation to be recorded is at a substantially zero value, saidapparatus comprising a light source, first means for directing a firstlight beam onto said information areas, means responsive to the clocksignal for modulating said first light beam in order to record digitallycoded information of fixed bit frequency in said information areas,first means for detecting radiation of said first beam which isreflected or transmitted by said medium, to produce a first electricalsignal representative of the information recorded in saidsynchronization areas and of said clock signal, first means forextracting said clock signal from said first signal, second means forprojecting a second light beam onto said track pattern behind said firstlight beam, second means for detecting radiation of said second beamwhich is reflected from or transmitted by said recording medium toproduce a second electrical signal representitive of the informationread by said second beam, second means for extracting said clock signalfrom said second signal, means for comparing the phase of said clocksignals extracted from said first and second signals, respectively, toproduce a control signal indicative of the phase differencetherebetween, adjustable delay means responsive to said control signalfor delaying the clock signal extracted by said second extracting meansby an amount such that the clock signal extracted thereby is in phasewith the clock signal extracted by said first extracting means when saidfirst beam scans said synchronizaton areas and means for applying saidclock signal to said modulating means.
 2. The apparatus according toclaim 1 including switching means for applying said control signal tosaid delay means and means for controlling said switching means suchthat said switching means is closed and said control signal is appliedto said delay means when said first beam scans said synchronizationareas and said switching means is open when said first beam scansinformation areas.
 3. The apparatus according to claims 1 or 2 whereinsaid first and second extracting means each includes a band pass filtertuned to said frequency of said periodic track modulation, each filterbeing connected in series with a respective one of said detecting means.